Immediately after a massive disaster, survival necesities are all that matter. People may not want to trade necessities for something that has no immediate use.
However, it won't always be that way. People will develop new systems for clean water, septic handling, making clothing, etc.
At that point, commerce will be at play, just like it has been for humanity in the past. People will use precious metals (and other things) as currency to get their needs met. Especially, since being 100% sustainable all by yourself is nearly impossible.
